Touch panels have recently become widely adopted as the input device for high-end portable electronic products such as smart-phones and tablet devices. Although, a number of different technologies can be used to create these touch panels, capacitive systems have proven to be the most popular due to their accuracy, durability and ability to detect touch input events with little or no activation force.
The most basic method of capacitive sensing for touch panels is the surface capacitive method—also known as self-capacitance—for example as disclosed in U.S. Pat. No. 4,293,734 (Pepper, Oct. 6, 1981). A typical implementation of a surface capacitance type touch panel is illustrated in FIG. 1 and comprises a transparent substrate 100, the surface of which is coated with a conductive material that forms a sensing electrode 110. One or more voltage sources 120 are connected to the sensing electrode, for example at each corner, and are used to generate an electrostatic field above the substrate. When an input object 130 that is electrically conductive—such as a human finger—comes into close proximity to the sensing electrode, a capacitor 140 is dynamically formed between the sensing electrode 110 and the input object 130 and this field is disturbed. The capacitor 140 causes a change in the amount of current drawn from the voltage sources 120 wherein the magnitude of current change is related to the distance between the finger location and the point at which the voltage source is connected to the sensing electrode. Current sensors 150 are provided to measure the current drawn from each voltage source 120 and the location of the touch input event is calculated by comparing the magnitude of the current measured at each source. Although simple in construction and operation, surface capacitive type touch panels are unable to detect multiple simultaneous touch input events as occurs when, for example, two or more fingers are in contact with the touch panel.
Another well-known method of capacitive sensing applied to touch panels is the projected capacitive method—also known as mutual capacitance. In this method, as shown in FIG. 2, a drive electrode 200 and sense electrode 210 are formed on a transparent substrate (not shown). A changing voltage or excitation signal is applied to the drive electrode 200 from a voltage source 220. A signal is then generated on the adjacent sense electrode 210 by means of capacitive coupling via the mutual coupling capacitor 230 formed between the drive electrode 200 and sense electrode 210. A current measurement means 240 is connected to the sense electrode 210 and provides a measurement of the size of the mutual coupling capacitor 230. When the input object 130 is brought to close proximity to both electrodes, it forms a first dynamic capacitor to the drive electrode 270 and a second dynamic capacitor to the sense electrode 280. If the input object is connected to ground, as is the case for example of a human finger connected to a human body, the effect of these dynamically formed capacitances is manifested as a reduction of the amount of capacitive coupling in between the drive and sense electrodes and hence a reduction in the magnitude of the signal measured by the current measurement means 240 attached to the sense electrode 210.
As is well-known and disclosed, for example in U.S. Pat. No. 7,663,607 (Hotelling, Feb. 6, 2010), by arranging a plurality of drive and sense electrodes in a grid array, this projected capacitance sensing method may be used to form a touch panel device. In such a system the location of touch input is determined by monitoring the capacitance changes at each intersection of drive electrode and sense electrode in the array. If the sensitivity of the projected capacitive touch sensor is sufficiently high, the measured capacitance may change considerably as the input object approaches, but does not touch, the touch panel surface. A threshold value of capacitance change is therefore defined such that when the measured change exceeds this threshold value the input object is considered to be touching the surface. An advantage of the projected capacitance sensing method over the surface capacitance method is that multiple simultaneous touch input events may be detected.
A limitation of the capacitance measurement techniques described above as conventionally applied to touch panels is that they are incapable of detecting input from non-conductive or insulating objects, for example made of wood, plastic or the like. Provided that a non-conductive object has a dielectric permittivity different to air it will cause the measured array capacitances to change when in close proximity to the touch panel surface. However, the magnitude of the resulting signal is very small—for example, less than 1% of that generated by a conductive object—and is dependent on the type of material the non-conductive object is made of and the ambient environment conditions. This disadvantageously reduces the usability of the touch panel since it is restricted to operation using conductive input objects, such as a finger or metallic pen or stylus. In particular, the user cannot operate a touch panel reliably while wearing normal (non-conductive) gloves or while holding a non-conductive object such as a plastic pen.
Although drops of water on the touch panel surface may be considered as non-conductive objects, the drops are not considered input objects under control of the user and their effect should therefore be rejected as opposed to detected. For example, US Patent Application 20040189617 (Gerpheide, Sep. 30, 2004) describes a capacitive touch panel that is capable of compensating for the effect of drops of water on the touch panel surface. The touch panel array includes an additional electrode to detect the presence of the non-conductive water droplets so that the touch panel may be used in wet conditions. The touch panel is not however capable of detecting the location of non-conductive input objects in general.
It is therefore desirable to provide a means of detecting both conductive and non-conductive types of input object using a capacitive type touch panel. Further, it is desirable to provide a means of distinguishing between conductive and non-conductive types of input object.